Method of forming permanent electrostatic image with two-layered photoreceptor

ABSTRACT

A PHOTOSENSITIVE MEMBER HAVING A TWO LAYERED PHOTOCONDUCTIVE PORTION WHICH COMPRISES A FIRST LAYER OF VITREOUS SELENIUM AND A SEOCND LAYER OVERLYING THE FIRST LAYER WHICH COMPRISES AN ARSENIC-SELENIUM ALLOY. THE MEMBER IS IMAGED BY UNIFORMLY ELECTROSTATICLALLY CHARGING THE SURFACE OF THE SECOND LAYER FOLLOWED BY UNIFORMLY EXPOSING THE CHARGED SURFACE TO RADIATION WITHIN THE RANGE OF ABOUT 4000 TO 6900 ANGSTROM UNITS.THE MEMBER IS THEN EXPOSED TO RADIATION WITHIN THE RANGE OF 7000 ANGSTROM UNITS TO 3 MICRONS WHICH RESULTS IN THE FORMATION OF A DEVELOPABLE LATENT ELECTROSTATIC IMAGE WHICH IS CONTAINED WITHIN THE PHOTOCONDUCTIVE PORTION OF THE IMAGING MEMBER.

g- 1972 A. .1. CIUFFINI 3,684,500

METHOD OF FORMING PERMANENT ELECTROSTATIC IMAGE WITH TWO-LAYERED PHOTQRECEPTOR Filed Dec. 18, 1970 I V ENTOR. ANTHONY J. CIUFFINI ATTORNEY United States Patent US. Cl. 96-1 R 8 Claims ABSTRACT OF THE DISCLOSURE A photosensitive member having a two layered photoconductive portion which comprises a first layer of vitreous selenium and a second layer overlaying the first layer which comprises an arsenic-selenium alloy. The member is imaged by uniformly electrostatically charging the surface of the second layer followed by uniformly exposing the charged surface to radiation within the range of about 4000 to 6900 angstrom units. The member is then exposed to radiation within the range of 7000 angstrom units to 3 microns which results in the formation of a developable latent electrostatic image which is contained within the photoconductive portion of the imaging member.

BACKGROUND OF THE INVENTION This application relates to xerography, and more specifically, to a novel method of imaging a photoconductive device.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate .is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely-divided electroscopic marking particles on the surface of the photoconductive insulating layer. This concept was originally described by Carlson in US. Pat. 2,297,691, and is further amplified and described by many related patents in the field.

When used in the conventional xerographic mode, a xerographic photoreceptor, which is normally in the form of a drum, is usually cycled through at least six basic steps. These include: (1) uniformly electrostatically charging the surface; (2) imaging the charge drum in the dark by exposure to a pattern of light which results in the formation of a latent electrostatic image on the drum surface; (3) developing the latent electrostatic image by cascading the drum with electroscopic toner particles which adhere to the drum surface to form a powder image; (4) transferring the powder image to a sheet of plain bond paper; (5) fusing the transferred image to the paper to form a permanent visible copy; and (6) cleaning the drum surface. In order to prepare a second duplicate image, all of the above six steps must be employed a second time.

It can therefore be seen that if a photoreceptor drum is to be used in a high speed duplicating manner, where the same image is copied a plurality of times, each of the above six steps must be repeated when using the conventional xerographic imaging mode. In repeating the above six steps a great many times, the photoreceptor will exhibit gradual deterioration due to repeated exposure to light, chemical solvents, abrasion, and heat. It can also be seen that the elimination of one or more of these steps would significantly increase the speed of the process and also add to the useful life of the drum.

An alternative method of xerography which uses special xerographic techniques, and usually a specially treated or constructed photoreceptor, involves the concept of persistent conductivity in the formation of useful images. Persistent conductivity may be defined as a latent image which exists as a state of electrical conductivity in a photoconductive layer, and persists as an after effect of the exposure radiation. An example of this phenomena is as follows: A photosensitive member which includes a layer of a suitable photoconductive insulator contained on a conductive substrate is exposed to an optical image in the form of pattern of light. The exposed image area becomes more or less conductive depending upon the amount of light impinging on the photoconductive surface during the exposure step. This conductivity pattern in the exposed area persists in the dark after exposure, while on the other hand, the background or dark areas remain relatively nonconductive.

One Way in which this concept may be used to form useful images is to first expose a photoconductive surface to an optical image which causes the formation of a latent conductivity image. Using a corona charging device, an electrical surface charge is then uniformly applied to the photoconductive surface. The areas previously exposed to light, which are of course conductive, dissipate the surface charge in the previously exposed areas, while the dark areas will retain a surface charge, thus forming an electrostatic image which is a reversal of the latent conductivity image. This electrostatic image may then be developed by any conventional developing technique used in xerography. One example of this technique is more fully defined in US. Pat. 3,427,157 in which a photoconductor which exhibits persistent conductivity comprises amorphous selenium containing about 0.001 to 5 weight percent thallium. In one embodiment, this structure is imaged by exposure to a pattern of light which forms a pattern of persistent conductivity on the photoconductor surface. The plate is uniformly charged to a negative polarity which results in a charge pattern being formed on the unexposed areas of the plate. The charge pattern is then developed with toner particles and the toner image transferred to a sheet of paper and fixed to form a permanent copy. In forming additional copies of the first image, the plate need only be charged again to retain a charge pattern in the unexposed areas. [It can be readily seen that this technique, unlike conventional xerography, allows a great number of duplicate copies of an original to be made without re-exposing the photoconductor layer for each cycle of operation.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a method of imaging which overcomes the above noted disadvantages.

It is another object of this invention to provide an improved method of imaging suitable for duplicating a plurality of the same image.

It is a further object of this invention to provide a novel method of imaging which allows for the storage of a developable image within a photoreceptor device for an indefinite period of time.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with the invention by providing a xerographic member having a composite photoreceptor portion. This member is particularly adaptable for use in a xerographic duplicating process in which multiple copies are made of the same image. As stated above, the member includes a two layered photoconductor portion. The first or base layer, which is usually contained on a conductive substrate, comprises a photoreceptor material which is sensitive to the visible spectrum in the range of about 4000 to 6900 angstrom units and typically includes such materials as vitreous selenium. The second or top photoconductive layer comprises a material which is sensitive to the near infrared and infrared portion of the spectrum and typically includes such materials as arsenic-selenium alloys. The structure is imaged by uniformly corona charging the surface to a given potential, followed by uniformly exposing the charged surface to visible light in the range of about 4000 to 6900 angstrom units, which injects the charge into the composite photoreceptor. At this point, most of the charge is trapped at some point within the composite photoreceptor with the field now being impressed across the composite photoreceptor and a supporting substrate, which is usually a conductor. The structure is then imaged with radiation capable of penetrating the top layer such as near infrared or infrared radiation in the range of about 7000 angstrom units to about 3 microns. This imaging radiation selectively discharges the member, which results in the formation of a latent electrostatic image within the composite photoreceptor portion of the member. Following the imaging step, the latent electrostatic image may be developed and the toner image transferred to a sheet of paper. This latent image, which is stored within the xerographic member, may now be developed and transferred as many times as desirable. After completion of a plurality of copies, or a copy run, image erasure is accomplished by simply uniformly exposing the plate to the same radiation which was used to form the latent image. At this point, the xerographic member is ready for immediate re-use.

The process of the present invention eliminates several conventional xerographic imaging steps which are necessary in carrying out a duplicating run in which the same image is copied a plurality of times. Y

The instant concept also provides advantages over the persistent conductivity techniques described above, in that here, the electrostatic image may be stored for an extremely long period of time without degradation of the image until it is ready to be used. In addition, recharging between making a duplicate image is not required. Further, when it is desired to change the image, one exposure to the proper radiation will discharge the plate and make it ready for a second imaging or duplicating cycle.

BRIEF DESCRIPTION OF THE DRAWING The advantages of the instant invention will become apparent upon consideration of the following disclosure of the invention, especially when taken in conjunction with the accompanying drawing, wherein:

The figure represents a schematic illustration of one embodiment of a xerographic member as contemplated for use in the instant invention.

DETAILED DESCRIPTION OF THE DRAWING In the drawing, reference character illustrates one embodiment of an improved photoreceptor device of the instant invention. Reference character 11 designates a support member which is preferably an electrically conduc tive material. The support may comprise a conventional metal such as brass, aluminum, steel or the like. The support may also be of any convenient thickness, rigid or flexible, and in any suitable form such as a sheet, web, cylinder, or the like. The support may also comprise other materials such as metallized paper, plastic sheets covered with a thin coating of aluminum or copper iodide, or glass coated with a thin conductive layer of chromium or tin oxide. An important consideration is that the support member be somewhat electrically conductive, or have a conductive surface or coating, and that it be strong enough to withstand a certain amount of handling.

Reference character 12 designates a photoconductive layer which is formed on support 11. This photoreceptor may typically comprise vitreous selenium or vitreous selenium containing a small amount of arsenic in the range of about 0.1 to 2.5 percent by weight. Overlaying photoreceptor layer 12, is a relatively thinner photoreceptor layer 13 which comprises a vitreous seleniumarsenic alloy containing arsenic in the range of about 15 to 30 weight percent.

The thickness of the first or lower photoreceptor layer is not particularly critical, and may vary from about 10 to 200 microns. Thicknesses in the range of 20 to microns are particularly satisfactory. The thickness of the top layer should be in the range of about 2 to 20 microns, but thicknesses outside this range can also be used.

The two photoreceptor layers of the instant invention may be prepared by any suitable technique. A preferred technique includes vacuum evaporation wherein each photoconductive layer is sequentially evaporated onto its corresponding base material. In this technique, the first layer, which may comprise selenium, and a second or top layer, which may comprise an arsenic-selenium alloy, are each evaporated by separate steps under vacuum conditions varying from about l0- to l0- torr. In another embodiment of this particular technique, the two photoreceptor layers may be continuously vacuum evaporated, one after the other, in the same vacuum chamber without breaking the vacuum, by sequentially activating two separate sources of selenium and arsenic-selenium.

Another typical technique which may be used includes co-evaporation, wherein the appropriate amount of each material for the alloy layers is placed in separate heated crucibles maintained under vacuum conditions with a source temperature of each alloy constituent being controlled so as to yield the appropriate percentage of the alloy desired.

Another typical method includes flash evaporation under vacuum conditions similar to those defined in co-evaporation, in which a powdered alloy such as selenium-arsenic alloy is selectively dropped into a heated crucible maintained at a temperature of about 350 to 600 C. The vapors formed by the heated mixture are evaporated up ward onto a substrate supported above the crucible.

In all of the above methods, the substrate onto which the photoconductive material is evaporated is maintained at a temperature of about 50 to 80 C. If desired, a water cooled platen or other suitable cooling means may be used in order to maintain a constant substrate temperature. In general, a selenium layer thickness of about 60 microns is obtained When vacuum evaporation is continued for about 1 hour at a vacuum of about 10- torr at a crucible temperature of about 280 C. U.S. Pats. 2,803,542 to Ullrich; 2,822,300 to Mayer et al.; 2,901,348 to Dessauer et al.; 2,963,376 to Schaffert; and 2,970,906 to Bixby all illustrate vacuum evaporation techniques which are suitable in the formation of the alloy layers of the instant invention. The crucibles which are used for the evaporation of the photoreceptor layers may be of any inert material such as quartz, molybdenum, stainless steel coated with a layer of silicon monoxide, or any other equivalent material. In general, the selenium or selenium alloy being evaporated as maintained at a temperature above about its melting point.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following examples further specifically define the present invention with respect to a method of making and imaging a composite photoreceptor suitable for use in a duplicating process sequence.

EXAMPLE Two stainless steel evaporation boats having a surface coating of silicon monoxide are placed in a vacuum evaporation chamber. The first boat contains an alloy of 99.5 weight percent xerographic grade vitreous selenium having a purity of about 99.999 percent and 0.5 weight percent arsenic available from Canadian Copper Refiners in the form of pellets ranging in diameter from about A to 05 inches. The second boat contains to 7 inch diameter pellets of an alloy containing 80 weight percent vitreous selenium and 20 weight percent arsenic. Each boat is connected directly to a source of electrical power adaptable to control the temperature of the respective boat. The surface of an oxidized aluminum drum 9.5 inches in diameter is placed approximately 6 inches above the two boats. The chamber is then evacuated to a vacuum of about 10- torr. The 99.5 percent selenium=0.5 percent arsenic containing boat is then heated to a temperature of about 285 C. for about 45 minutes to form a layer of vitreous selenium about 50 microns thick on the aluminum plate. The drum is rotated at the rate of about 20 revolutions per minute during the evaporation cycle. This boat is then covered with a metal shutter and the elec trical power to the boat turned off. The boat containing the 80 percent selenium=20 percent arsenic alloy is then heated to a temperature of about 350 C. for minutes to form a 10 micron thick layer of the selenium-arsenic alloy over the first photoconductive layer. At the end of this time, the vacuum chamber is cooled to room tempera ture, the vacuum broken and the composite photoconductive plate removed from the chamber.

The drum of the example is first charged to a positive potential of 800 volts. The drum is then uniformly exposed to radiation in the range of about 4000 to 6500 angstrom units which results in the surface charge being injected into the top layer and being trapped within the composite photoreceptor. The plate is then exposed to an image in the form of a pattern of infrared radiation in the range of about 7000 angstrom units to about 3 microns using an incandescent lamp. This results in the selective discharge of the charge contained within the plate and forms a developable latent electrostatic image. The image is then developed by cascading electroscopic toner material over the surface and transferring the powder image to a sheet of paper and fixing to form a permanent image. This imaging sequence is reported four additional times with image quality being equal for all five copies. The plate is then made ready for a second image by uniformly exposing the plate to a source of infrared radiation which erases the first latent image.

It should be pointed out that if either photoconductive layer is doped with a halogen, the novel method of the instant invention cannot be carried out in that the composite photoreceptor will not store the latent image.

Although specific components and proportions have been stated in the above description of the preferred embodiment of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials and changes may be utilized which synergize, enhance, or otherwise modify the photoreceptor.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading this disclosure. These are also intended to be within the scope of this invention.

What is claimed is:

1. A method of imaging which comprises:

(a) providing a photosensitive member having a two layered photoconductor portion which comprises a first layer of vitreous selenium and a second layer overlaying said first layer which comprises an arsenicselenium alloy, with the arsenic concentration of the second layer being in the range of about 15 to 20 weight percent with the balance substantially selemum;

(b) uniformly electrostatically charging the surface of the arsenic-selenium layer to a positive potential;

(c) uniformly exposing the charged surface to radiation within the range of about 4000 to 6900 angstrom units; and

(d) exposing said member to a pattern of imaging radiation within the range of about 7000 angstrom units to 3 microns which results in the formation of a developable latent electrostatic image which is contained within the photoconductive portion of the imaging member.

2. The method of claim 1 in which the latent electrostatic image is developed to form a visible image.

3. The method of claim 2 in which the imaging and developing sequence are repeated at least one additional time.

4. The method of claim 3 in which after making a plurality of images, the member is uniformly exposed to infrared radiation which results in the erasure of the latent electrostatic image.

5. The method of claim 1 in which the two photoconductor layers are contained on a supporting substrate.

6. The method of claim 5 in which the substrate is electrically conductive.

7. The method of claim 1 in which the selenium layer is about 10 to 200 microns in thickness and the arsenic-selenium layer is about 2 to 20 microns thick.

8. The method of claim 1 in which the vitreous selenium of the first layer contains arsenic in an amount of about 0.1 to 2.5 percent by weight.

References Cited UNITED STATES PATENTS 3/1970 Regensburger 117-215 4/ 1967 Straughan 96-15 US. Cl. X.R. 101-456, 458 

